CBSE · Class 12 · Chemistry · Coordination CompoundsExplain Werner's Coordination Theory of coordination compounds with its main postulates. How did Werner explain the difference between the electrical conductivity and precipitation behavior of $\text{CoCl}3 \cdot 6\text{NH}3$, $\text{CoCl}3 \cdot 5\text{NH}3$, and $\text{CoCl}3 \cdot 4\text{NH}3$ when treated with excess silver nitrate solution?

Introduction to Werner's Coordination Theory\nAlfred Werner put forward his theory of coordination compounds in 1893 to explain the nature of bonding in coordination entities. He proposed the concept of primary and secondary valency.

Main Postulates of Werner's Theory

• Two Types of Valencies: Metals exhibit two types of valencies in coordination compounds: primary valency and secondary valency.
• Primary Valency (Ionizable): This corresponds to the oxidation state of the metal. It is ionizable and satisfied by negative ions (anions). It is non-directional.
• Secondary Valency (Non-ionizable): This corresponds to the coordination number of the metal. It is non-ionizable, satisfied by neutral molecules or negative ions, and is directed towards fixed positions in space, determining the geometry of the complex.
• Arrangement in Space: Ions/groups bound by secondary valencies have characteristic spatial arrangements corresponding to different coordination numbers (e.g., octahedral, tetrahedral, square planar).

Explanation of Cobalt Ammine Complexes\nWerner tested his theory by studying the reaction of cobalt(III) chloride ammine complexes with excess silver nitrate ($\text{AgNO}_3$) solution to precipitate chloride ions as silver chloride ($\text{AgCl}$).

1. $\text{CoCl}_3 \cdot 6\text{NH}_3$ (Formulated as $[\text{Co(NH}_3)_6]\text{Cl}_3$):

   • Primary valency = 3, Secondary valency = 6.
   • Ionization gives 3 chloride ions per molecule.
   • Reaction with excess $\text{AgNO}_3$ precipitates 3 moles of $\text{AgCl}$ per mole of the compound.

2. $\text{CoCl}_3 \cdot 5\text{NH}_3$ (Formulated as $[\text{Co(NH}_3)_5\text{Cl}]\text{Cl}_2$):

   • Primary valency = 3, Secondary valency = 6 (5 $\text{NH}_3$ and 1 $\text{Cl}^-$ inside the coordination sphere).
   • Ionization gives 2 chloride ions per molecule.
   • Reaction with excess $\text{AgNO}_3$ precipitates 2 moles of $\text{AgCl}$ per mole of the compound.

3. $\text{CoCl}_3 \cdot 4\text{NH}_3$ (Formulated as $[\text{Co(NH}_3)_4\text{Cl}_2]\text{Cl}$):

   • Primary valency = 3, Secondary valency = 6 (4 $\text{NH}_3$ and 2 $\text{Cl}^-$ inside the coordination sphere).
   • Ionization gives 1 chloride ion per molecule.
   • Reaction with excess $\text{AgNO}_3$ precipitates 1 mole of $\text{AgCl}$ per mole of the compound. \nThis clearly demonstrated that primary valencies are ionizable and outside the coordination sphere, while secondary valencies are bound firmly inside.